1. Field of the Invention
The present invention relates to medical instruments and methods for performing eye surgery to correct irregularities of the cornea. More particularly, the present invention relates to mechanical instruments known as microkeratomes, and related surgical methods for performing lamellar keratotomies.
2. The Related Art
The first microkeratome for performing corneal resections was developed in 1962 by one of the present inventors, Doctor Jose I. Barraquer, and is shown generally in FIG. 1A. This microkeratome includes a guide ring which is fixed to an ocular globe, or eyeball, with the aid of a partial vacuum applied through the ring. The guide ring immobilizes the ocular globe, maintains the tension of the globe, and regulates the diameter of the corneal resection. A portion of the microkeratome called a cutting head is supported within a channel in the guide ring for guided linear movement of the microkeratome across the ring by the surgeon. The cutting head carries a cutting blade that is oscillated by a motor-driven eccentric transverse the channel as the instrument is moved through the cutting path defined by the channel. The cutting head carries a removable, lower plant member that compresses the ocular globe ahead of the oscillating blade, to permit the blade to cut a lamella having a lower surface that is parallel to the surface of the cornea that is compressed by the planar member. The planar member is interchangeable with similar planar members of differing thicknesses, so as to vary the thickness of the resectioned corneal "disk."
Numerous variations on the Barraquer microkeratome have been made since 1962, including the apparatus that is the subject of U.S. Pat. No. 4,662,370 assigned to Carl-Zeiss-Stiftung of Germany. The '370 patent describes a microkeratome having interchangeable inserts with convex, concave, and planar surfaces that engage and compress the cornea for producing a corneal resection of predetermined form and curvature. The inserts are set within a stationary planar member that is fixed to the guide ring. The cutting blade is moved through a cutting path parallel to the planar member defined by a gap between the planar member and the guide ring, and oscillates transverse the path.
While apparently effective to permit resections of corneal lenticula, the apparatus of the '370 patent lacks means for controlling, or automating the rate of movement by the cutting head across the guide ring, and is therefore prone to binding up in the corneal tissue, or otherwise producing imprecise resections under unsteady progress by the surgeon's hand. Furthermore, there is no apparent means for changing the depth or thickness of the corneal resection. Also, this apparatus is limited to use in lamellar keratectomies (excision of a corneal section), as opposed to lamellar keratotomies (incision through the cornea).
The problem of controlled movement across the guide ring has been addressed by the instrument described in U.S. Pat. No. 5,133,726, which has been reissued as Re 35,421, to Luis A. Ruiz and Sergio Lenchig G. The '726 and '421 patents disclose a microkeratome, shown in FIG. P2, having a gear transmission assembly for moving the instrument through the cutting path at a controlled rate of speed. The gears are driven by the same motor that drives the cutting blade and engage a track atop the guide ring. Thus, the automated transmission system is an improvement over the instrument of the '370 patent, but in practice it has been found that the weight of the motor in the instrument produces a large moment through the handle of the device. This moment, coupled with the forward positioning of the gear that engages the guide ring track, causes the rear surface of the cutting head to bind in its engagement with the guide ring. At best, this results in uneven travel by the instrument during the surgery and unnecessary pressure fluctuations within the eye. At worst, such binding can cause irregular cutting of the cornea that produces leucoma, or the induction of an astigmatism.
The relatively recent technological development of intrastromal refractive surgery led to the creation of instruments and methods for performing incomplete lamellar temporo-nasal keratotomies, which leave a peripheral residue of corneal tissue uncut to act as a "nasal hinge." The nasal hinge permits the corneal disk to be lifted for exposure and carving of the stromal layer, such as by a laser. The use of a laser to perform stromal carving in association with an incomplete lamellar keratotomy is referred to as "Laser Intrastromal Keratomileusis" ("LASIK").
In similar fashion to the original Barraquer device, the microkeratome of the '726 and '421 patents include a forward planar member in the lower portion of the cutting head that is interchangeable with similar planar members of varying thicknesses. For the planar member to be interchangeable, however, a slotted portion of the cutting head extends substantially forward of the cutting blade to receive the planar member. This, and the fact that the transmission gears are positioned outside the cutting head, result in a fairly large surface area, or "footprint" for the instrument. The large footprint restricts the manner in which the microkeratome can be used, and generally requires that it be moved across the cornea from the temporal region adjacent the eye, producing the vertical nasal hinge when performing incomplete lamellar keratotomies. The vertical nasal hinge has at least two deficiencies. First, the corneal disk resulting from the LASIK, or other procedure, will be vertically displaced after surgery, and/or pleated to some extent by the opening and closing of the upper eyelid. Second, the formation of a vertical nasal hinge on the corneal disk increases the likelihood of accidental ablation of the hinge during the correction of an astigmatism, which is typically performed with vertical cutting motions across a major diameter of the cornea.
The large surface area of the planar member, or plaque, described in the '726 patent is designed to substantially compress the entire cornea at any one time. Such action produces unnecessarily high intraocular pressure, which unduly stresses the eye and could result in complications during surgery. Furthermore, the interchangeable planar member lack means for indicating the thickness of the resection to be provided by the respective members. This creates the possibility that a planar member having the wrong thickness will be inserted into the cutting head. In such an event, the instrument might perform an exaggerated cut and perforate the ocular globe, causing serious consequences.
In response to the shortcomings described herein, it is an object of the present invention to provide an improved microkeratome wherein the cutting head is moved across the cornea by sweeping pivotal motion relative to a fixed point on a guide ring positioned on a patient's eye.
It is a further object that such pivotal motion be induced automatically by the engagement of gears with an output shaft that induces rotation about a pivot post connected to a guide ring at the fixed point.
It is a further object to provide an improved microkeratome having an adjustable float head, or plaque, connected to the cutting head for varying the thickness of corneal resections without having no disassemble or replace a component of the instrument.
It is a further object that the adjustable float head be provided with indicia for indicating the selected depth of cut, whereby inadvertent mistakes regarding the thickness of the resection are less likely to occur.
It is a further object of the present invention that the float head exhibit a small surface area to permit corneal resections without the cutting head or float head exceeding the rim of the guide ring. In this manner, oblique and lower-upper resections of the cornea can be performed during a lamellar keratotomy, whereby the risk of inadvertent ablation of the corneal hinge during an astigmatic correction, as well as the extent of fold and displacement of the corneal disc generated by the sweeping of the upper eyelid, are reduced.
It is a further object that the float head impart only the minimum pressure upon the ocular globe that is necessary to compress the cornea for a uniform resection.
It is a further object that the transmission gears be positioned inside the side walls of the cutting head to minimize the surface area of the instrument, whereby a superior corneal hinge lying in the upper region of the cornea can be produced.
It is a further object that, in a linearly driven embodiment of the present invention, the drive gear which engages the guide ring be positioned rearwardly with respect to the cutting head to eliminate any binding between the rear lower surface of the cutting head and the guide ring.
It is a still further object of the present invention to provide a means for selectively limiting the cutting range of the microkeratome through the cutting path defined by the guide ring, so as to regulate the formation of the corneal hinge during a lamellar keratotomy.
It is a still further object to provide means for automatically returning the microkeratome to its initial position on the guide rings, upon reaching the selected limit of the cutting range.